Sean K. Ellis spent nearly 22 years in prison for the 1993 murder of Boston Det. John Mulligan. Ellis claimed innocence, and in 2015 the courts overturned his conviction, ruling "justice was not done" (unanimously upheld by the Mass. SJC in 2016). A retrial -his 4th- is scheduled for Oct. 18, 2018.

MURDER AND INVESTIGATION-1993

"A message"

Fifty-two-year-old Boston Detective John Mulligan was shot five times in the face just before dawn on Sunday, September 26, 1993, as he slept in his SUV outside an all-night Walgreens drugstore in Boston's Roslindale section. Someone either fired through his slightly open driver's window or climbed into his passenger seat to make the kill. No windows were broken, and no one heard the shots. Observers discerned a cross-like pattern in the bullets. "There were some elements that would lead one to describe it as execution-style...an assassination," said Police Commissioner William Bratton. It sure looked personal, Boston pundits agreed, "a message." Who would fire that many shots at a uniformed policeman?

"Problem officer"

A 27-year veteran of the force, Mulligan had a rough policing style that earned him scores of enemies and multiple lawsuits for false arrest and citizen brutality. The year before his death he was officially branded a "problem officer" by his department, one of a "handful of officers with the largest number of civilian complaints against them."

Random crime

To the public's surprise, within days the police dragnet turned up two teenagers from adjacent Boston neighborhoods, Terry Patterson, age 18, of Hyde Park, and Sean Ellis, age 19, of Dorchester. The authorities relabeled Mulligan's brutal assassination a random crime of opportunity, committed when the youths saw the detective sleeping and decided to steal his gun for a trophy.

Many observers were skeptical: Would teens kill a uniformed officer -- and kill so brutally -- on a motive so whimsical? And was the killing truly random, given the victim's scores of enemies and his gangland-style execution?

Detectives Kenneth Acerra (L) and his partner, Walter Robinson, lead Sean Ellis to jail after his October 1993 arraignment. Both detectives were soon accused -- by prosecutors and defense lawyers alike -- of mishandling evidence and witnesses in the Ellis case. Five months after Ellis's conviction, a Boston Globe exposé unveiled the partners as corrupt, and in 1998 they pleaded guilty to more than 40 federal counts of perjury and armed robbery in a decade-long criminal scheme of targeting drug dealers and illegal immigrants for profit. (Boston Herald photo; all rights reserved.)

Police misconduct?

After Patterson and Ellis's arrests, allegations hit the press of possible investigative misconduct -- charges all the more surprising because they came from the Suffolk County DA's office. Prosecutors questioned two incidents:

First, John Mulligan's friend and colleague, Detective Kenneth Acerra, claimed to have "found" Mulligan's cell phone -- reported missing after the murder -- in the center console of Mulligan's Ford Explorer a full week after the crime. Acerra claimed he was looking for a charger when he came upon the cell phone -- even though crime scene investigators who swept the vehicle had not found it. The explanation strained credulity.

A second questionable incident also involved Acerra, this time with his partner, Detective Walter Robinson, and a close associate, Detective John Brazil. Within twelve hours of the murder, the three detectives brought forward nineteen-year-old Rosa Sanchez, who claimed that while shopping for a bar of soap at Walgreens at 3 a.m., she saw a young African-American man peering into the sleeping Mulligan's car window.

Rosa Sanchez, coincidentally, was the niece of the woman Acerra lived with.

Sanchez went on to identify Sean Ellis from a police photo array containing eight images, but only after homicide detectives granted her two separate tries. In her first viewing of the photos, she selected another man-- neither Ellis nor Patterson -- and was escorted out of the building by Acerra and Robinson. (The photo she selected was the same that Rosa's husband, Ivan Sanchez, had earlier selected from the same arrays in a private viewing at his home.)

Outside, Rosa and her young husband sat in her "Uncle Kenny" Acerra's private car with both him and Walter Robinson; by all accounts she was weeping.

Moments later, Rosa Sanchez was brought back into homicide by Acerra and Robinson. Shown the unchanged photo array a second time, she immediately pointed to Sean Ellis's photo.

Suspicious over Acerra and Robinson's investigative actions, Chief Prosecutor Phyllis Broker first removed Acerra from the task force over the cell phone incident. (The powerful Detectives Union got him quickly reinstated). Next, the very night of Rosa Sanchez's two-try photo ID, Broker required both Acerra and Robinson to answer questions on tape about their roles in the unorthodox procedure.

The powerful Detectives' Union strongly and publicly objected to Broker's actions as "harassment" of the detectives, and the ensuing feud between Boston Police and the DA's office hit the press. According to the Boston Globe, Boston Police Commissioner William Bratton then called in Prosecutor Broker and told her that, if she wanted to keep her job, there would be no further questioning of Acerra and Robinson.

Broker complied.

The Sanchez photo ID of Ellis was admitted as evidence over the vigorous objections of Ellis's trial lawyers.

Free at last, June 3, 2015. Sean Ellis leaves Boston's Suffolk Superior Court holding his mother, Mary's, hand. Arrested at age 19, he spent more than half his life in prison for a crime he maintains he did not commit. (Getty Images)